Canon CP-E4 Compact Battery Pack: Batteries in your Speedlite 580EX II, MT-24EX and MR-14EX flash units will usually provide more than enough power for most uses but sometimes you want to take lots of pictures with flash, the flash needs to recycle really quickly, you might want to use a high power burst of flash frequently. The Canon CP-E4 battery pack provides an extra power source to help you to do this.